6.14.2011

In the Citi breach, the data thieves were able to penetrate the bank’s defenses by first logging on to the site reserved for its credit card customers.

Once inside, they leapfrogged between the accounts of different Citi customers by inserting various account numbers into a string of text located in the browser’s address bar. The hackers’ code systems automatically repeated this exercise tens of thousands of times — allowing them to capture the confidential private data.

The method is seemingly simple, but the fact that the thieves knew to focus on this particular vulnerability marks the Citigroup attack as especially ingenious, security experts said.